Why Does AC Turn On With Defrost On Car?

If you have ever had to get ready for a drive on a cold winter morning then you have probably experienced what happens. After you scrape all that snow and ice off the outside of your car and then hop inside to try to get warm. Your front windshield is probably all fogged up and you’ll need to sit there with the defroster running for a good 5 minutes. So before it can clear up enough so that you can safely pull out of your driveway and head out on the road.

The defroster at the front of your car uses the same system that your air conditioner and heater use to either keep you cool or warm as needed. That means the same blower motor, the same vent system, and the same fan. The only real difference between your defroster and your heater is the direction that the air is going to be blowing when you turn it on. Follow the article to know why does ac turn on with defrost.

How Does A Defroster Work?

Your car defroster is tied into the air conditioning system. While that means it can get quite warm and very cold, it also means something else. In case you have ever had to use a humidifier in your home during the winter. Because your furnace was removing too much moisture from the air, you’re already familiar with what’s going on here.

Your air conditioner whether it’s set to cold or hot condenses the moisture out of the air into water. This condensation is vented through a drain hose that runs from behind your glove box out the bottom of the car. The system then pumps dry air into the car. When you turn on the defroster, it pumps dry air up toward the windshield. This helps to evaporate the moisture.

Types of Car Defrosters

There are two primary kinds of defrosters. The first type uses a vehicle’s heating, ventilation, and air conditioning (HVAC) systems to blow warm, dehumidified air directly onto the foggy or iced-over windshield. The other type of defrosting system defogs and de-ices through a mechanism known as resistive heating.

1. Primary Car Defrosters 

Defrosters that use a vehicle’s HVAC system are sometimes referred to as primary defrosters. Since they are designed to clear up the front and side windows, and they work via two main principles.

To melt ice that has accumulated on the windshield. The HVAC system activates the primary defroster to draw in the fresh air, passing it through the vehicle’s heater core. It then directs the warm air through dashboard vents toward the front windshield and side windows.

In addition to defrosting windows, these primary systems can also defog windows by removing condensation from the inner surface. To accomplish this, a front window defroster will typically pass air through the air conditioning system to remove moisture. When this dehumidified air reaches a fogged windshield, it absorbs moisture and removes the condensation.

Warm air can also hold more moisture than cold air, which is what makes primary defrosters effective when these two systems work in concert. Although it is possible to accomplish the same moisture removal process by physically wiping the condensation away, doing so may leave smudges that can result in glare. It may also make it difficult to see through the windshield at times.

2. Secondary Car Defrosters

Defrosters that don’t use a car’s HVAC system are sometimes referred to as secondary systems since they are designed to defrost things like rear windshields and mirrors. These systems typically use wire grids and resistive heating to physically warm up the surface of the glass. This can effectively melt ice and remove condensation.

Rear windshield defrosters typically use surface-mounted grids that are easily identifiable. While heated mirrors usually have internal wires that you can’t see. However, both systems use the same basic mechanism of resistive heating. An electrical current is applied to the wire grid when you activate the system. And the resistance of the grid causes heat generation.

Why Does AC Turn On With Defrost?

This occurs due to different reasons. But there is a solution, just follow the below steps to get rid of it.

1. Turn Your Heater On

Start your engine, and using the defroster setting, crank the heater up all the way to absorb excess moisture within your vehicle. You should remember hot air can hold more moisture. This is the first step to know why does ac turn on with defrost.

2. Press The A/C Button

This may sound counterintuitive to the whole heater thing. But really what you are doing is activating a setting on your car’s A/C system. This helps dry the air within the cab faster with the help of the coils in your A/C system.

3. Turn Air Recirculation Off

You need fresh air to enter the car as well. Winter air is cold, and as it does not hold much moisture, it is dry. Bringing it into your car ups absorption capacity to more quickly dry the saturated air trapped within.

4. Crack Your Windows

This helps exchange the humid air in your car with dryer outside air, speeding up the process.

5. Defrost Windows

While the above defogging action is taking place, de-frost your windows. This is an entirely separate process that should never include hot water. Defrosting quickly takes a minor amount of pre-planning, but saves a snow-capped mountain of headache later. In case you can not stand scraping, put together a simple solution of 2/3 cup Isopropyl or rubbing alcohol and 1/3 cup water, pouring the mixture into a spray bottle.

This solution will not freeze and can be stored easily in your car. Allowing you to easily spray away windshield ice wherever you’re stranded. Follow the above steps to know why does ac turn on with defrost.

How Much Does It Cost To Fix The Defrost On A Car?

You will pay about 15$ to 30$ for the parts to replace the rear window defroster on your car. The labor should be 30$ to 50$, in most cases. As it is a quick and simple job to fix a connection issue, which is the most common problem with rear defrosters.

The repair kits are readily available and inexpensive. So if you are going to do the repairs yourself, you could save on the minor costs associated with taking the vehicle to a professional mechanic.


No matter what symptoms your air conditioning may be experiencing. A qualified automotive technician knows how to properly diagnose car AC problems. How to properly restore your air conditioning’s function.

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